Current industrial and military applications often use Yttrium-Iron-Garnet (YIG) filters to tune circuits in high-frequency applications such as waveguide filters in microwave communications. The major drawback to this approach is the slow tuning speeds, 200 to 400 microseconds, due to the ferrite hysteresis of the magnetic tuning circuit. Also, the high magnetic fields require large, expensive and heavy magnetic field coils and complex high-current drivers.
Mechanically tuned capacitors, an alternative to the YIG filter, are implemented by inserting screws into a waveguide. Filters using mechanically tuned capacitors allow wide-range tuning of the filter's pass band. Unfortunately, screws physically inserted into the waveguide are seen by different frequencies to have widely different sizes. Because the screw size is seen differently by different frequencies, the pass band center frequency varies nonlinearly. Also, to tune a waveguide filter, the screws controlling the capacitance must be adjusted, a time-consuming mechanical process often requiring a remote controlled actuator. Such filters cannot be adjusted rapidly, since adjustment of the screws requires time and mechanical energy. As a result of the delays imposed by the mechanical tuning and as a result of the nonlinear relationship between center frequency and capacitance, electronic tuning commands generated by computers are difficult to implement effectively.
To satisfy high-speed tuning requirements, some designs now utilize varactors, which can be tuned at tuning-rates three orders of magnitude higher than those of the YIG. Because varactors take advantage of the voltage dependence of the capacitance across the charge separation in the depletion region of a p-n junction, varactors may be tuned in a capacitance range determined by the range of available voltages, from a reverse voltage of zero volts to breakdown. But varactors have very high losses, particularly at higher frequencies, which must be compensated. Active devices are available to compensate some of this loss, but only over a narrow tuning range. The negative resistance provided by an active element such as a MESFET or other high-Q Gallium-Arsenide active device, for example, can provide an impedance with the necessary negative real part, but at a cost of tuning range. In other words, although the varactor itself can be tuned over a range of voltages from zero to breakdown, the high-frequency losses limit the usefulness of varactor-tuned filters over narrow frequency ranges. Since tuning range varies with band, at high frequency bands another approach is needed if selectivity and tuning speed are not to be sacrificed.